mapWithIndex<B> method
Implementation
@override IMap<K, B> mapWithIndex<B>(B f(int i, V a)) =>
_tree.foldLeft<Tuple2<int, IMap<K, B>>>(tuple2(0, emptyMap()), (t, k, v) => t.apply((i, acc) => tuple2(i+1, acc.put(k, f(i, v))))).value2;